In some instances the tubular string downhole has to be cut such as when a tool is stuck and needs to be fished out. Other tools, like packers, have thin portions that are designed to be cut as the manner for release. When the mandrel is cut and compromised, the set force that holds the slips engaged to the tubular wall is released to allow the slips to retract so that the packer can be removed. Regardless of what type of tubular structure is being cut there is a reason to be concerned about the depth of cut since there are often structures in close proximity to the tubular being cut that could be damaged if the cutting went out radially too far.
Cutting devices such as underreamers are frequently used to cut downhole tubulars. They feature a plurality of arms that are radially extendable with cutting structure on the arms. A hydraulic motor turns the arms to cut the tubular. Surface personnel look at the pressure to the motor and look for a rapid change marking the removal of load as the tubular in question is cut all the way through. However, in many cases, the underreamer when getting through the tubular that it was cutting continues to turn and go to its outward maximum dimension. While surface personnel may detect a pressure change in the hydraulic drive system there can still be further damage done downhole before they can respond to such a change and shut the surface pumps off to retract the blades of the underreamer. The present invention addresses a way to avoid damage to surrounding structures that can be damaged by too deep a cut by an underreamer or some other rotary cutting tool. In essence, the cutter is prevented from further cutting once it extends to a predetermined dimension. In the preferred embodiment, a loose fitting sleeve is placed behind the tubular wall to be cut. Once the wall is through the spinning blades engage the rotating sleeve as cutting action stops. Instead the underreamer or cutter simply spins the sleeve and exhibits a low or no load hydraulic condition on the drive motor giving surface personnel a clue that the tubular has been cut through and that the tool can be retracted and removed without damage to structures beyond the rotating sleeve.
Underreamers with a device to absorb the initial impact of contact with the tubular to be cut and to control the cutting rate and amount of cutter contact by being rubbed down or off are illustrated as item 37 in U.S. Pat. No. 7,036,611. Also relevant to this art are U.S. Pat. Nos. 7,308,937 and 6,702,031.
Those skilled in the art will have a better understanding of the invention from the detailed description and associated drawings that appear below with the understanding that the full scope of the invention is given by the literal and equivalent scope of the appended claims.